Light-emitting diodes (LEDs) are gradually replacing incandescent light bulbs in various applications, including traffic signal lamps, large-sized full-color outdoor displays, various lamps for automobiles, solid-state lighting devices, flat panel displays, and the like. Conventional LEDs typically include a light-emitting semiconductor material, also known as the bare die, and numerous additional components designed for improving the performance of the LED. These components may include a light-reflecting cup mounted below the bare die, a transparent encapsulation (typically silicone) surrounding and protecting the bare die and the light reflecting cup, and electrical leads for supplying the electrical current to the bare die. The bare die and the additional components are efficiently packed in an LED package.
The advent of blue- and ultraviolet-emitting LEDs has enabled the widespread deployment of LED-based white light sources for, e.g., general lighting applications and backlights for liquid crystal displays. In many such light sources, a portion the high-frequency light of the LED is converted to light of a different frequency, and the converted light combines with unconverted light to form white light. Yellow-emitting phosphors have been advantageously combined with blue LEDs in this manner. One popular configuration for LEDs and phosphors is the “remote-phosphor” arrangement, in which the phosphor and the LED are spatially separated to (1) maintain the phosphor at a lower temperature during LED operation and thereby improves efficiency of the phosphor and (2) reduce the amount of light that is backscattered from the phosphor and absorbed by the LED itself (which lowers the overall efficiency of the device).
Planar remote-phosphor LED-based devices have additional advantages. In such devices, the phosphor is typically located at a greater distance from the LEDs and is thus exposed to much lower temperatures during operation, and light reflected from the phosphor may not propagate back to the light-absorbing LED. In addition, planar devices are very attractive due to their slim appearance; in contrast to LEDs, which are point sources of light, a planar device emits light from a larger area.
FIG. 1 schematically depicts a conventional planar remote-phosphor LED lighting device 100, in which the LED 110 is spatially separated from a phosphor layer 120 on a waveguide 130. In some configurations, scattering elements (e.g., located at the bottom surface of waveguide 130) disrupt the total-internal-reflection (TIR) confinement of light within waveguide 130 so that the light may be emitted through the phosphor layer 120. (As used herein, TIR confinement refers to confinement of light due to the index-of-refraction difference between the confining waveguide and the surrounding ambient, e.g., air, rather than via an opaque reflector.) While the distance between the LED 110 and the phosphor layer 120 improves illumination efficiency, as described above, this configuration does have disadvantages. First, as shown in FIG. 1, the phosphor layer is often applied to the exit surface of the waveguide (as that is typically the farthest point from the LED), but the exit surface is often quite large. Thus, a large amount of phosphor material, which is typically exotic and/or expensive, is required. For example, the planar lighting device 100 has a large exit surface that requires a significant amount of phosphor in the coating phosphor layer 120. This results in low utilization of the phosphor (in terms of light intensity emitted per amount of phosphor in the coating), which may be expensive. Second, since the particular LED/phosphor combination in the lighting device constrains the choice of suitable phosphor materials, the lighting device may require use of a phosphor material that has an undesirable color when the lighting device is in the off state (i.e., not emitting light). For example, many conventional phosphors have yellow and/or green hues that dictate the color of (at least a large portion of) the lighting device itself in the off state. In many applications it may be desirable for the lighting device to have a different (or even controllable) appearance in the off state. Thus, there is a need for remote-phosphor lighting devices that utilize less phosphor material without significantly impacting performance and the off-state color of which may be controlled and/or unconstrained by the color of the phosphor material itself. Furthermore, such devices preferably have a slim geometry and also minimize the amount of light reflected back from the remote phosphor into the LED itself, which tends to absorb such light and reduce overall efficiency.